Project Summary The circuitry and function of sensory cortices, including auditory cortex, are sensitive to changes in experience and sensory environment. Neurons in layer 1 (L1) of cortex have a privileged position in controlling the activity and plasticity of cortex induced by salient experiences. L1 contains a small number but diverse class of inhibitory neurons that nearly all express the ionotropic serotonin 5-HT3A receptor. These neurons are known to increase cortical activity by inhibiting other inhibitory GABAergic interneurons. Nevertheless, the mechanisms by which L1 neurons are activated and control cortical state are not fully understood. We unexpectedly found that, in primary auditory cortex (A1), L1 neurons receive a direct and tonotopically-organized input from auditory thalamus. Furthermore, we made the surprising discovery that L1 neurons send inhibitory projections back to thalamus. Here we propose to use anatomical, electrophysiological, and viral approaches to elucidate the pre and post-synaptic partners of 5-HT3A-expressing neurons in L1 of A1, focusing on the possibility of a closed loop from thalamic relay neurons to L1 neurons and back. Our findings suggest that classical models of cortico-thalamic interactions are incomplete and indicate that L1 receives tuned sensory input and contains projection neurons. These studies will lay the foundation of a future RO1 examining the functional effects of this novel inhibitory cortico-thalamic projection on the activity and plasticity of A1 circuits.